Why does the deamination of cytosine to uracil not cause a problem in the next r

Question

Why does the deamination of cytosine to uracil not cause a problem in the next round of DNA replication?

A) Although uracil cannot form the third hydrogen bond to guanine, it is recognized by DNA polymerase as a mismatch and the error is corrected on the next round of replication.

B) The AP endonuclease nicks the backbone and removes the uracil and replaces it with cytosine.

C) The repair machinery recognizes uracil in DNA as a mistake and replaces it with cytosine.

D) The uracil undergoes spontaneous depurination and is replaced with cytosine.

E) Deamination of cytosine rarely occurs within the coding region and so causes few mutations.

Explanation / Answer

Uracil is a demethylated form of thymine. Cytosine deaminates to form uracil. The deamination of cytosine is mutagenic as uracil pairs with adenine and one of the daughter strands will contain an U-A base pair not as the original C-G base pair.

It is prevented by a repair system that recognizes uracil as a foreign to DNA. The enzyme, uracil DNA glycosylase, is homologous to AlkA which hydrolyzes the glycosidic bond between the uracil and deoxyribose and does not attack thymine-containing nucleotides.

The AP site generated is repaired and it reinsert cytosine.

So answer is B

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